it isn't needed for PropertyExpressionInterpreter because it's the default implementation.
imo we don't need such an indirection right now, because it's mainly an extension point for concrete projects and we won't have that many alternatives in our code-base. currently it's just SystemPropertyExpressionInterpreter. -> it would be enough to move it to org.apache.myfaces.extensions.cdi.core.api.interpreter
(as soon as we need such an indirection (it's never needed by users for custom implementations), we can add it easily).